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1 perry 1 pgmcrater
2 !!!pgmcrater
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 OPTIONS
7 DESIGN NOTES
8 SEE ALSO
9 AUTHOR
10 ----
11 !!NAME
12
13
14 pgmcrater - create cratered terrain by fractal forgery
15 !!SYNOPSIS
16
17
18 __pgmcrater__ [[__-number__ ''n'']
19 [[__-height__|__-ysize__ ''s'']
20 [[__-width__|__-xsize__ ''s''] [[__-gamma__
21 ''g'']
22
23
24 All options can be abbreviated to their shortest unique
25 prefix.
26 !!DESCRIPTION
27
28
29 __pgmcrater__ creates a PGM image which mimics cratered
30 terrain. The PGM image is created by simulating the impact
31 of a given number of craters with random position and size,
32 then rendering the resulting terrain elevations based on a
33 light source shining from one side of the screen. The size
34 distribution of the craters is based on a power law which
35 results in many more small craters than large ones. The
36 number of craters of a given size varies as the reciprocal
37 of the area as described on pages 31 and 32 of Peitgen and
38 Saupe[[1]; cratered bodies in the Solar System are observed
39 to obey this relationship. The formula used to obtain crater
40 radii governed by this law from a uniformly distributed
41 pseudorandom sequence was developed by Rudy
42 Rucker.
43
44
45 High resolution images with large numbers of craters often
46 benefit from being piped through __pnmsmooth__. The
47 averaging performed by this process eliminates some of the
48 jagged pixels and lends a mellow ``telescopic image'' feel
49 to the overall picture.
50
51
52 __pgmcrater__ simulates only small craters, which are
53 hemispherical in shape (regardless of the incidence angle of
54 the impacting body, as long as the velocity is sufficiently
55 high). Large craters, such as Copernicus and Tycho on the
56 Moon, have a ``walled plain'' shape with a cross-section
57 more like:
58
59
60 /\ /\
61 _____/ ____________/____________/ _____
62 Larger craters should really use this profile, including the central peak, and totally obliterate the pre-existing terrain.
63 !!OPTIONS
64
65
66 __-number__ ''n''
67
68
69 Causes ''n'' craters to be generated. If no
70 __-number__ specification is given, 50000 craters will be
71 generated. Don't expect to see them all! For every large
72 crater there are many, many more tiny ones which tend simply
73 to erode the landscape. In general, the more craters you
74 specify the more realistic the result; ideally you want the
75 entire terrain to have been extensively turned over again
76 and again by cratering. High resolution images containing
77 five to ten million craters are stunning but take quite a
78 while to create.
79
80
81 __-height__ ''height''
82
83
84 Sets the height of the generated image to ''height''
85 pixels. The default height is 256 pixels.
86
87
88 __-width__ ''width''
89
90
91 Sets the width of the generated image to ''width''
92 pixels. The default width is 256 pixels.
93
94
95 __-xsize__ ''width''
96
97
98 Sets the width of the generated image to ''width''
99 pixels. The default width is 256 pixels.
100
101
102 __-ysize__ ''height''
103
104
105 Sets the height of the generated image to ''height''
106 pixels. The default height is 256 pixels.
107
108
109 __-gamma__ ''factor''
110
111
112 The specified ''factor'' is used to gamma adjust the
113 image in the same manner as performed by __pnmgamma__.
114 The default value is 1.0, which results in a medium contrast
115 image. Values larger than 1 lighten the image and reduce
116 contrast, while values less than 1 darken the image,
117 increasing contrast.
118
119
120 Note that this is separate from the gamma correction that is
121 part of the definition of the PGM format. The image
122 __pnmgamma__ generates is a genuine, gamma-corrected PGM
123 image in any case. This option simply changes the contrast
124 and may compensate for a display device that does not
125 correctly render PGM images.
126 !!DESIGN NOTES
127
128
129 The __-gamma__ option isn't really necessary since you
130 can achieve the same effect by piping the output from
131 __pgmcrater__ through __pnmgamma__. However,
132 __pgmcrater__ performs an internal gamma map anyway in
133 the process of rendering the elevation array into the PGM
134 format, so there's no additional overhead in allowing an
135 additional gamma adjustment.
136
137
138 Real craters have two distinct morphologies.
139 !!SEE ALSO
140
141
142 pgm(5), pnmgamma(1),
143 pnmsmooth(1)
144
145
146 [[1]
147
148
149 Peitgen, H.-O., and Saupe, D. eds., The Science Of Fractal
150 Images, New York: Springer Verlag, 1988.
151 !!AUTHOR
152
153
154 John Walker
155 Autodesk SA
156 Avenue des Champs-Montants 14b
157 CH-2074 MARIN
158 Suisse/Schweiz/Svizzera/Svizra/Switzerland
159 Usenet:
160 kelvin@Autodesk.com
161 Fax:
162 038/33 88 15
163 Voice:
164 038/33 76 33
165
166
167 Permission to use, copy, modify, and distribute this
168 software and its documentation for any purpose and without
169 fee is hereby granted, without any conditions or
170 restrictions. This software is provided ``as is'' without
171 express or implied warranty.
172
173
174 __PLUGWARE!__ If you like this kind of stuff, you may
175 also enjoy ``James Gleick's Chaos--The Software'' for
176 MS-DOS, available for $59.95 from your local software store
177 or directly from Autodesk, Inc., Attn: Science Series, 2320
178 Marinship Way, Sausalito, CA 94965, USA. Telephone: (800)
179 688-2344 toll-free or, outside the U.S. (415) 332-2344 Ext
180 4886. Fax: (415) 289-4718. ``Chaos--The Software'' includes
181 a more comprehensive fractal forgery generator which creates
182 three-dimensional landscapes as well as clouds and planets,
183 plus five more modules which explore other aspects of Chaos.
184 The user guide of more than 200 pages includes an
185 introduction by James Gleick and detailed explanations by
186 Rudy Rucker of the mathematics and algorithms used by each
187 program.
188 ----
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